A. Field of the Invention
The present invention relates to a lockup damper included in a lockup mechanism of a torque converter for damping a vibration transmitted from an input rotary member to an output rotary member.
B. Description of the Background Art
In general, the damper mechanism transmits a torque from an input rotary member to an output rotary member, and simultaneously operates to damp a vibration transmitted from the input rotary member toward the output rotary member. A lockup mechanism, which is included in a lockup mechanism disposed inside the torque converter and will be referred to as a "lockup damper", is an example of the above damper mechanism.
The torque converter is internally provided with three kinds of vane wheels, i.e., an impeller, a turbine and a stator, and is operable to transmit a torque through a working fluid filling an internal space thereof. The impeller is fixed to a front cover coupled to the input rotary member. The working fluid flowing from the impeller to the turbine through the stator transmits a torque from the impeller to the turbine, and then is transmitted to the output rotary member coupled to the turbine.
The lockup mechanism is disposed between the turbine and the front cover for mechanically coupling the front cover and the turbine together and thereby directly transmitting the torque from the input rotary member to the output rotary member.
Usually, the lockup mechanism has a piston member which can be pressed against the front cover, a retaining plate fixed to the piston member, coil springs carried by the retaining plate and a driven member elastically coupled, in a rotating direction of the mechanism, to the piston member through the coil springs. The driven member is fixed to the turbine coupled to the output rotary member. The components of the lockup mechanism also form a lockup damper mechanism for absorbing and damping an applied vibration.
When the lockup mechanism operates, the piston member slides on or is pressed to the front cover so that the torque is transmitted from the front cover to the piston member, and then is transmitted to the turbine through the coil springs. The lockup mechanism transmits the torque, and also operates to absorb and damp the torsional or angular vibration owing to the lockup damper. The coil springs are repetitively compressed between the retaining plate fixed to the piston member and the driven member, and thereby slide on the retaining plate so that the vibration is damped.
Recently, such a lockup damper which includes elastic members arranged at a radially outer portion of a torque converter having a relatively large space has been used in many cases for reduction in axial distance of the torque converter. However, arrangement of the elastic members at the radially outer portion of the torque converter reduces an allowable maximum torsion angle of the torque converter compared with the structures including elastic members arranged at the radially middle or inner portion. For overcoming the above disadvantage, two elastic members may be arranged in series with an intermediate member or the like therebetween. The elastic members connected in series can be compressed by a large length, so that an intended maximum torsion angle can be ensured. Combination of two elastic members having different spring constants can provide the torsion characteristics of the lockup damper having two stages, and therefore can improve the torsion characteristics.
In the above lockup damper, it is necessary to provide a stop or stopper mechanism for restricting a relative rotation between the drive and driven members through an angle larger than a predetermined angle. Thus, a relative rotation between the drive and driven members must be inhibited by the operation of the stop mechanism when a torque larger than a predetermined value is transmitted. This stop mechanism may be formed of coil springs, which are employed as elastic members and can function as the stop mechanism when they are fully compressed.
However, the coil springs to be used also as the stop mechanism must have a sufficiently large endurance strength against the maximum torque load which may be transmitted, so that specifications of the coil springs cannot be selected from a wide range. This results in restriction of the damper characteristics and increase in cost of the coil springs. Recently, it has been desired to improve the durability of the lockup mechanism. Therefore, a load against the coil spring must be reduced.
If the elastic members (coil springs) are not used as the stop mechanism, an independent stop mechanism is required, which increases the numbers of parts and manufacturing steps as well as the manufacturing cost.